This invention relates to finned tubing. The invention relates in particular to a method of and apparatus for manufacturing bimetal finned tubing and to finned tubing manufactured in accordance with the method.
More specifically, the invention relates to the manufacture of bimetal finned tubing from tubular metal fin stock on a tubular metal liner by means of a plurality of rollers drivably mounted on arbors spaced about the tubular fin stock. Each of such rollers includes a plurality of axially arranged forming discs and the arrangement is such that the discs on the respective rollers track with each other. The discs can be brought into rolling contact with the fin stock and when this is done and the arbors are driven, the discs penetrate into the fin stock and deform the stock so that the metal in the fin stock is forced into axial spaces between the forming discs to thereby form fins on the tube liner. The axes of the arbors intersect the axis of the tubular fin stock so that when the arbors are driven the forming discs traverse the fin stock longitudinally and thereby extrude helical fins from the fin stock. Due to pressures exerted by the forming discs on the fin stock during the extrusion of the fins, the fins thus formed engage the tube liner frictionally.
According to the invention there is provided a method of manufacturing finned tubing from tubular metal fin stock on a tube liner by means of a plurality of rollers rotatably drivable by means of arbors and each roller including a plurality of axially arranged forming discs, the method including deforming the metal of the fin stock to form a fin by forcing the metal into a restricted radially directed axial space defined between adjacent forming discs so that the metal is uninterruptedly in contact with a portion of the axially directed face of one disc defining the restricted space but is only in partial contact with the opposing axially directed face of the adjacent forming disc defining the restricted space, there being no contact over a radially extending portion spaced radially inwardly from the disc periphery.
At least some of the discs may be resiliently flexible. Thus the method may further include applying lateral pressure in an axial direction to the fin by deflection of a forming disc. During the application of lateral pressure to the fin, the fin may be inclined with respect to the axis of the tube liner..
Further according to the invention there is provided an apparatus for manufacturing finned tubing from tubular metal fin stock on a tube liner, the apparatus including a forming disc which is mountable on a rotatably drivable arbor so as to be rotatable in a direction transverse to the rotational axis of the arbor, the disc having a cross-sectional profile of which a radially extending portion is recessed in the axial direction of the disc with respect to the profile regions which are radially adjacent to the recessed region.
The disc may be resiliently flexible in its axial direction and may be capable of applying lateral pressure in the axial direction of the disc to a fin formed from the fin stock. Thereby the fin may be inclined with respect to the axis of the tube liner.
The recessed portion in the cross-sectional profile of a forming disc in accordance with the invention may be provided in one axial side only of the disc, while the other axial side of the disc may have a regular, generally linear or curved profile. Alternatively, recessed portions in the profile may be provided in both axial faces of the forming disc.
The recessed portion may be formed by a smoothly curved region on the forming disc intersecting a linear portion. The curved portion may be provided inwardly of the periphery of the disc and may be so shaped that the profile initially diverges from the periphery of the disc in a radially inward direction and then converges, whereafter the profile again diverges radially inwardly. The arrangement is such that a tangent extending between the prominence on the curved portion of the profile formed between the divergent and convergent regions bridges the recessed region before it contacts a radially inward portion of the disc profile. This arrangement ensures that metal forced into a gap defined on one side by the recessed portion will not contact the forming disc along the recessed portion.
At least some of the discs may be dish-shaped when not in use and may be mounted so that their dished faces are directed against the direction in which lateral pressure is applied to the fin, the lateral pressure tending to deform the discs, and the dished shapes of the fins tending to resist such lateral pressure and deformation under the lateral pressure.
Furthermore, at least some of the discs may have asymmetrical cross-sectional profiles.
Still further, the recessed portion of the disc may be provided in the dished surface.
The cross-sectional profile of the disc in the face in which the recessed portion is provided may include an annular shoulder at the radially inward extremity of the profile.
The periphery of the disc may be a sharp edge or it may be blunt and may be in the form of a peripherally extending flat surface.
In a particular embodiment the thickness of the disc may be 1.6 mm.
The discs may be provided in at least two groups, the discs in one group effecting penetration of the fin stock and initial forming of the fin and another group effecting final forming and finishing of the fin. The second group of discs may effect radially extending the fin and forcing the fin back onto the tube liner.
The discs may be provided in two groups by positioning a stiff, relatively inflexible restraining disc between the two groups, the restraining disc resisting deformation of the two groups of discs under lateral pressure applied to the discs in opposite directions.
The diameters of the forming discs in the first and the second groups may gradually increase. The angle of a line extending axially over the peripheries of the diametrically increasing discs with respect to the axis of the tube liner may be 6.degree. in the first group and about 1.degree. in the second group.
The angle of inclination of the arbor axis with respect to the tube liner axis determines the pitch of the helical fin. The lead angle may be example be 30 minutes (half of a degree), and this may result in a helical fin having a pitch of eleven fins per 25 mm (one inch) i.e. a pitch of 2.3 mm. The arbor may be rotated at 730 rpm. At this rotational speed a fair amount of heat is generated due to friction, and in order to alleviate this a lubricant/coolant may be used. A suitable lubricant/coolant may be a substance including a fatty acid.
The invention further extends to a roller for manufacturing finned tubing from tubular metal fin stock on a tube liner, the roller including a plurality of discs mounted in an axial arrangement on an arbor in which at least some of the discs are in accordance with the apparatus of the invention.
In one arrangement, the discs may be mounted so that they abut each other axially.
Alternatively, spacers may be provided between the discs so that the discs are thereby axially separated from each other.
The discs may be provided in at least two groups, the discs in one group effecting penetration of the fin stock and initial forming of the fin and another group effecting final forming and finishing of the fin.
A stiff, relatively inflexible restraining disc may be positioned between the two groups, the restraining disc resisting deformation of the two groups of disc under lateral pressure applied to the discs.
Both groups may include dished discs and may be so mounted that their dished faces are directed towards the restraining disc.
At least four of the discs immediately adjacent either axial side of the restraining disc have blunt peripheries in the form of flat peripheral surfaces.
Conveniently, the roller may include from 31 to 33 discs.
A roller in accordance with the invention may include a plurality of forming discs in several groups, the cross-sectional profiles of the forming discs in the several groups being different, but the cross-sectional profiles of the discs in any one group being substantially the same.
There may conveniently be provided four groups of forming discs in the roller. The first group may have sharp wedge-shaped peripheries to facilitate penetration of the tubular fin stock. The discs in the next group may have cross-sectional profiles similar to those in the first group, but the profiles may be less sharp and may be slightly curved and may also be of gradually increasing thickness in order to gradually compress the metal between the discs. The discs in the third group may have cross-sectional profiles with recessed portions in accordance with the invention for forming the fin stock and gradually raising the fin. The peripheries of these discs may be slightly curved. The fourth group of discs may have cross-sectional profiles similar to those of the third group but their peripheries may be sharper. The discs in this group perform a finishing action on the extruded fin. Finally, a buttress disc is provided which is resiliently flexible and applies lateral pressure in an axial direction to the fin.
The invention still further extends to finned tubing whenever made in accordance with the method of the invention.
The tube liner and the fin stock may be different metals. The tube liner may for example be steel, more particularly carbon steel, while the fin stock may be aluminium.